Block copolymer based on polymer having thiol end group and linked by divalent sulfur

ABSTRACT

The present invention relates to a polymer having a thiol group at one end of the molecule, which is obtained by subjecting the monomers capable of radical polymerization to radical polymerization in the presence of a thiolcarboxylic acid and then treating the resulting polymer with an alkali or acid; to a process for producing the same; and to a block copolymer based on the polymer having a thiol group at the end. 
     The polymer having a thiol group at one end is difficult of forming disulfide linkages by oxidation and thus difficult of becoming insoluble. It is also a highly reactive material that will find wide use in varied application areas. The block copolymer based on the polymer having a thiol group at the end is superior in compatibility and will find wide use in varied application areas.

This is a division of application Ser. No. 592,476, filed Mar. 22, 1984,now U.S. Pat. No. 4,565,854.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a polymer having a thiol end group, aprocess for producing the same, and a block copolymer based on thepolymer having a thiol end group. The polymer is obtained by subjectingmonomers capable of radical polymerization to radical polymerization inthe presence of a thiolcarboxylic acid and then treating the resultingpolymer with an alkali or acid.

2. Description of the Prior Art:

A thiolcarboxylic acid ester is an important precursor for the synthesisof mercaptans and the introduction of a thiol group, because it iseasily decomposed by a base or acid producing a thiol group as a result.

A thiolcarboxylic acid ester is also important for the synthesis of apolymer having thiol groups.

There are proposed some methods for the synthesis of a polymer havingthiol groups, as in "Kobunshi Jikken 6" (Experiments in Polymers), pages367 to 371. According to these methods, vinyl monomers having athiolcarboxylic acid ester in the side chain undergo radicalpolymerization and then the resulting polymer is treated with a base oracid so that the thiol group is formed; or a thiolcarboxylic acid isadded to the double bond in polyisoprene or polybutadiene to form athiolcarboxylic acid ester and subsequently the ester is decomposed by abase or acid to form the thiol group.

The thiol groups in a polymer form the secondary structure through theformation of disulfide linkage. This structure plays an important rolein activity as observed in proteins and enzymes containing cysteine.Thus it is an extremely interesting subject in biochemistry.

Also in the field of synthetic polymers, many attempts have been made tosynthesize a polymer having thiol groups in the side chains. Such apolymer is expected to have an ability to perform oxidation andreduction or an ability to capture heavy metals through the formation ofmercaptide. Another effort was directed to the modification of polymersby the polymeric reaction which is induced by the high reactivity of thethiol group. In practice, however, a polymer having thiol groups has notbeen put to industrial use, because the thiol group is readily oxidizedto form a disulfide linkage and the disulfide linkage causescrosslinking, making the polymer insoluble.

SUMMARY OF THE INVENTION

The present inventors carried out researches on the production of apolymer having highly reactive thiol groups which is not insolubilizedby oxidation. It was found that such a polymer can be produced asfollows: Monomers capable of radical polymerization undergo radicalpolymerization in the presence of a thiolcarboxylic acid. Thethiolcarboxylic acid works as a chain transfer agent, and it is attachedto the end of the polymer in the form of thiolcarboxylic acid ester.When the polymer is treated with an alkali or acid, the thiolcarboxylicacid ester is decomposed. Thus there is obtained a polymer having athiol end group. The present invention was completed based on thisfinding.

The present inventors also found that when monomers capable of radicalpolymerization undergo radical polymerization in the presence of thepolymer having a thiol end group, a variety of block copolymers havinggood compatibility can be oatained. The present invention was alsocompleted based on this finding.

The thiolcarboxylic acid used in this invention includes organicthiolcarboxylic acids having the --COSH group. Where the thiolcarboxylicacid is represented by R--COSH, R denotes a C₁₋₁₈, preferably C₁₋₅,aliphatic hydrocarbon group such as an alkyl or aromatic hydrocarbongroup. Examples of thiolcarboxylic acids include thiolacetic acid,thiolpropionic acid, thiolbutyric acid, and thiolvaleric acid.Thiolacetic acid is preferable because of its decomposability.

DETAILED DESCRIPTION OF THE INVENTION

There are no restrictions as to the monomers which undergo radicalpolymerization in the presence of a thiolcarboxylic acid, so long asthey are capable of radical polymerization. Examples of such monomersinclude vinyl esters such as vinyl acetate, vinyl formate, vinylpropionate, vinyl laurate, and vinyl stearate; α-olefins such asethylene, propylene, and isobutylene; acrylic acid, methacrylic acid,and esters thereof such as methyl acrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate, dodecyl acrylate, 2-hydroxyethylacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate,2-ethylhexyl methacrylate, dodecyl methacrylate, 2-hydroxyethylmethacrylate, dimethylaminoethyl acrylate, dimethylaminoethylmethacrylate, and quaternary products thereof; acrylamide monomers suchas acrylamide, methacrylamide, N-methylolacrylamide,N,N-dimethylacrylamide, and acrylamide-2-methylpropane sulfonic acid andsodium salt thereof; aromatic vinyl monomers such as styrene,α-methylstyrene, and p-styrene-sulfonic acid and sodium salt andpotassium salt thereof; acrylonitrile and methacrylonitrile;N-vinylpyrrolidone; and halogen-containing vinyl monomers such as vinylchloride, vinyl fluoride, vinylidene fluoride, and tetrafluoroethylene.They may be used individually or in combination with one another.

Vinyl ether, allyl ether, allyl ester, and the like, which are incapableof or difficult of radical polymerization when used individually, may beused as a copolymer component. However, dienes, which form a polymerhaving double bonds, are not preferable, because the thiol formed by thedecomposition of a thiolcaroxylic acid ester is added to the double bondin the polymer.

Preferable among the above-mentioned monomers capable of radicalpolymerization are vinyl esters, acrylic acid, methacrylic acid, acrylicesters, methacrylic esters, acrylamide, methacrylamide, acrylonitrile,methacrylonitrile, vinyl halides, aromatic vinyl monomers, andα-olefins. Most suitable among them are vinyl esters, acrylic esters,methacrylic esters, acrylic acid, methacrylic acid, acrylamide,methacrylamdie, and vinyl halides.

According to this invention, the monomers capable of radicalpolymerization are polymerized in the presence of a thiolcarboxylicacid, and the radical polymerization can be accomplished by bulkpolymerization, solution polymerization, pearl polymerization, oremulsion polymerization in the presence of a radical polymerizationinitiator. A suitable method should be selected according to theproperties of the monomer and the intended polymer. There are norestrictions as to the quantity of a thiolcarboxylic acid to be added tothe polymerization system and as to the method of adding athiolcarboxylic acid to the polymerization system. A proper selectionshould be made according to the desired properties of the intendedpolymer. The polymerization may be performed batchwise, semibatchwise,or continuously.

The radical polymerization initiator that can be used in this inventionincludes the ordinary initiators such as 2,2'-azobisisobutyronitrile,benzoyl peroxide, and carbonate peroxide. The azo-initiator, such as2,2'-azobisisobutyronitrile is preferable because of its ease ofhandling. Radiation and electron rays can also be used as the initiator.The polymerization temperature is usually 10° to 90° C., depending onthe kind of initiator used.

After the polymerization is complete, unreacted monomers are removedfrom the resulting polymer having a thiolcarboxylic acid ester at theend, and the polymer is treated with an alkali or acid so that thethiolcarboxylic acid ester is decomposed. Thus there is obtained apolymer having a thiol end group.

The alkali or acid treatment should preferably be performed in a solventthat dissolves or swells the polymer. The use of such a solvent ispreferable for the control of reaction rate and selectivity. The alkalior acid treatment can be carried out with any one of sodium hydroxide,potassium hydroxide, ammonia, dimethylamine, diethylamine, hydrochloricacid, sulfuric acid, acetic acid, etc. Selection should be madeaccording to the chemical properties of the polymer. For example, astrong alkali such as sodium hydroxide is not suitable for apolyacrylate. It decomposes the ester in the polymer as well as thethiolcarboxylic acid ester at the end. Thus the polymer is not obtainedas intended. In such a case, a weak alkali such as ammonia should beused under a proper condition so that the ester of the polymer is notdecomposed. The selective decomposition of a thiolcarboxylic acid esterat the end can be accomplished by properly selecting the solvent,alkali, acid, reaction temperature, reaction time, etc. according to theindividual polymers.

The polymer having a thiol end group is not restricted in thecomposition, degree of polymerization, and molecular weightdistribution. They should be properly established according to theintended use of the polymer. The polymer obtained as mentioned above isa new, highly reactive material that will find use in a broadapplication area. The reactivity of the thiol end group will make itpossible to add the polymer to a compound having double bonds, toreplace the end group with a halogen compound, and to apply the polymerto redox reaction in combination with an oxidizing agent.

In addition, the polymer having a thiol end group is utilized for theproduction of a block copolymer by radical polymerization which isperformed in the presence of the polymer. (This will be describedlater.)

In what follows, a detailed description is made of the polymer having athiol end group produced by polymerizing vinyl esters as the monomerscapable of radical polymerization. The polymer is represented by theformula P.SH, wherein P contains at least one kind of the constituentunits A and B represented by the following formulas. ##STR1## (whereinR¹ is hydrogen or a C₁₋₆ hydrocarbon group, and R² is hydrogen or aC₁₋₂₀ hydrocarbon group.) This polymer is designated as X hereinafter.

In an embodiment (X₁) of the polymer (X), the content of B is not lessthan 50 mol % and not greater than 100 mol %, and the degree ofpolymerization is lower than 3500.

The polymer (X₁) is produced by polymerizing vinyl monomers composedmainly of vinyl esters in the presence of a thiolcarboxylic acid, andthen treating the resulting polymer with ammonia or an amine.

The vinyl ester polymer thus obtained has a thiol group at only one end.Therefore, it does not become insoluble even when the disulfide linkageis formed by oxidation, unlike the conventional polymer having thiolgroups which are randomly distributed in the main chain of the molecule.

Examples of the constituent unit A include vinyl alcohol, α-methylvinylalcohol, α-ethylvinyl alcohol, α-propylvinyl alcohol, α-butylvinylalcohol, and α-hexylvinyl alcohol. Examples of the constituent unit Binclude vinyl acetate, vinyl formate, vinyl propionate, vinyl laurate,and vinyl stearate, and α-substituted products thereof.

It is essential that the vinyl ester polymer contain the constituentunit B or both of the constituent units A and B. It may contain otherconstituent units than A and B so long as the content of B is greaterthan 50 mol %. The content of the constituent unit B should preferablybe greater than 70 mol % for adequate solubility in organlc solvents.

Examples of other constituent units than A and B include the units ofethylene, propylene, isobutene, acrylic acid, methacrylic acid, saltsand alkyl esters thereof, acrylonitrile, methacrylonitrile, acrylamide,methacrylamide, trimethyl(3-acrylamide-3-dimethylpropyl)ammoniumchloride, ethyl vinyl ether, butyl vinyl ether, N-vinyl pyrrolidone,vinyl chloride, vinyl fluoride, vinylidene chloride, vinylidenefluoride, tetrafluoroethylene, and other vinyl esters than theconstituent unit B.

The vinyl ester polymer is not restricted in the degree ofpolymerization. However, it should be lower than 3500, preferably lowerthan 2000, more suitably lower than 1000, the lower limit being about10, if the thiol end group is to function effectively. The vinyl esterpolymer will be effectively used in the application of the conventionalvinyl ester polymer such as a paint vehicle; but it will find new usesas a functional material owing to the reactivity of the thiol end group;for example, addition to a compound having double bonds, substitutionreaction with a halogen compound, and combination with an oxidizingagent for redox decomposition reaction. The special polymer compositionof vinyl ester polymer having a thiol group at only one end thereof isutilized for polymerizing monomers capable of radical polymerization inthe presence of the vinyl ester polymer to form a block copolymer inwhich the vinyl ester polymer constitutes one component.

The process for producing the vinyl ester polymer haivng a thiol endgroup has the following features. That is, a vinyl monomer mainlycomposed of vinyl ester such as vinyl acetate is polymerized in thepresence of a thiolcarboxylic acid to give a vinyl ester polymer havinga thiolcarboxylic acid ester group at the end, and the polymer issubsequently treated with ammonia or an amine. In this way, thethioester linkage at the end is decomposed to thiol but most of thevinyl ester units in the main chain remain undecomposed.

Any vinyl ester can be used so long as it is capable of radicalpolymerization. Examples of vinyl esters include vinyl formate, vinylacetate, vinyl propionate, vinyl ester of Versatic acid, vinyl laurate,and vinyl stearate. Preferable among them is vinyl acetate because ofits polymerizability and stability. These vinyl esters may becopolymerized with the above-mentioned copolymerizable monomers.

Various methods may be used for treating with ammonia or an amine thevinyl ester polymer having a thiolcarboxylic acid ester group at theend. For a reasonable reaction rate and selectivity, the treatmentshould preferably be carried out in a solvent that dissolves or swellsthe vinyl ester polymer. Examples of such a solvent include methanol,ethanol, acetone, dioxane, benzene, and toluene. Most suitable amongthem is methanol. It is inexpensive, it permits smooth reaction, and itworks as a good solvent for polymerization. Examples of amines includemethylamine, ethylamine, propylamine, dimethylamine, diethylamine,triethylamine, and other alkylamines; ethanolamine, propanolamine,triisopropanolamine, and other alkanolamine; and aniline,dimethylaniline, and other aromatic amines. However, treatment withammonia water, or liquid or gaseous ammonia is industrially preferable.

The reaction temperature and time should be suitably establishedaccording to the kind of amine to be used. They should be moderate inorder that the thiolcarboxylic acid ester at the end is selectivelydecomposed and the ester linkage in the main chain remains undecomposed.The preferred reaction temperature is 20° to 60° C.

When the reaction is complete, the polymer is discharged in the usualway such as precipitation with a non-solvent. The discharged polymershould preferably be washed with a non-solvent such as water andpurified by reprecipitation with water from an acetone solution.

In another embodiment (X₂) of the polymer (X), the content of A is notgreater than 100 mol % and not less than 50 mol %, and the degree ofpolymerization is lower than 3500, preferbly lower than 2000, and moresuitably lower than 1000, the lower limit being about 10 but with norestriction.

The vinyl alcohol polymer (X₂) having a thiol end group is produced bypolymerizing vinyl monomers composed mainly of vinyl esters in thepresence of a thiolcarboxylic acid, and then saponifying the resultingvinyl ester polymer.

The vinyl alcohol polymer thus obtained has a thiol group at only oneend. Therefore, it does not become insoluble even when the disulfidelinkage is formed by oxidation, unlike the conventional polymer havingthiol groups which are randomly distributed in the main chain of themolecule.

This vinyl alcohol polymer is represented by the above-mentioned formulaP.SH. It contains the constituent unit A or both of the constituentunits A and B. The content of B should be greater than 50 mol %, andpreferably greater than 70 mol % for adequate solubility in water. Theconstituent units A and B and other constituent units than A and B arethose which are described above for the vinyl ester polymer (X₁) havinga thiol end group. The polymerization is carried out in the same way asmentioned above.

The vinyl ester polymer having a thiolcarboxylic acid ester at the endis subsequently saponified in the usual way. Saponification is usuallycarried out in an alcohol, particularly methanol. Both absolute alcoholand water-containig alcohol can be used, and the alcohol may containmethyl acetate, ethyl acetate, and other organic solvents. Thesaponification temperature should be 10° to 70° C. The preferredcatalyst for saponification is sodium hydroxide, potassium hydroxide,sodium methylate, or potassium methylate. The quantity of the catalystis determined according to the degree of saponification and the quantityof water. It is more than 0.001 mol, preferably more than 0.002 mol for1 mol of the vinyl ester unit; and the upper limit is 0.2 mol. When usedexcessively, the alkaline catalyst remains unremoved in the polymer andcauses discoloration. In the case where the vinyl ester polymer containscarboxyl groups or esters thereof which react with an alkali, thequantity of the alkaline catalyst should be increased accordingly.

As the result of the saponification reaction, the end of the polymerchain changes into a thiol group and the main chain of the polymerchanges into vinyl alcohol. The degree of saponification of the vinylester unit in the main chain may be changed according to the intendeduse of the polymer. The polymer that separates out after saponificationshould be purified by washing with methanol to remove the alkali metalsalt of acetic acid. After drying, the polymer is obtained in the formof white powder.

The vinyl alcohol polymer having a thiol end group will be effectivelyused for the application of a conventional vinyl alcohol polymer such aspaper coating and textile finishing; but it will find new uses as afunctional material owing to the reactivity of the thiol end group. Forexample, it may be added to a compound having double bonds, it may beused to substitute a halogen compound, and it may be combined with anoxidizing agent for redox decomposition reaction. The special polymercomposition of the vinyl alcohol polymer having a thiol group at onlyone end is utilized for polymerizing monomers capable of radicalpolymerization in the presence of the vinyl alcohol polymer to form ablock copolymer in which the vinyl alcohol polymer constitutes onecomponent.

In what follows, a detailed description is made of the block copolymerof the present invention wherein the components are linked by divalentsulfur moieties which is obtained by subjecting monomers capable ofradical polymerization to radical polymerization in the presence of thepolymer having a thiol end group.

The polymer having a thiol end group which is used for producing theblock copolymer can be obtained in any one of the following ways.According to the first method, a polymer having hydroxyl groups at theends is subjected to chemical reaction so that the hydroxyl groups arechanged into thiol groups. For example, the two terminal hydroxyl groupsof polyethylene glycol or polypropylene glycol are changed into halidesor sulfonic acid esters, and then thiourea is added to form anisothiuronium salt, and finally the isothiuronium salt is hydrolyzedwith an alkali. According to the second method, the polymer is formed bycondensation polymerization of a dithiol and a diamine or diisocyanate.According to the third method, monomers capable of radicalpolymerization undergo radical polymerization in the presence of athiolcarboxylic acid such as thiolacetic acid, and the resulting polymeris treated with an alkali or acid, whereby a thiol group is introducedinto one end of the polymer molecule. For example, vinyl ester, methylmethacrylate, styrene, or the like undergoes radical polymerization inthe presence of a thiolcarboxylic acid, and the resulting polymer istreated with an alkali or acid, whereby there is obtained polyvinylester, polyvinyl alcohol, polymethyl methacrylate, or polystyrene havinga thiol end group. (The third method has been described in theforegoing.) There are another methods for introducing a thiol group intothe end of the polymer molecule. According to the present invention, norestrictions are made on the method of synthesis, and any polymer havinga thiol end group can be used. The one obtained according to the thirdmethod can be most effectively used in this invention.

The above-mentioned method for producing the block copolymer has afeature that radical polymerization is carried out in the presence of apolymer having a thiol end group. The radical polymerization can beaccomplished by bulk polymerization, solution polymerization, pearlpolymerization, or emulsion polymerization. A suitable method should beselected according to the desired properties of the polymer. Thepolymerization may be performed batchwise, semibatchwise, orcontinuously. There are no restrictions as to the method and process ofpolymerization so long as the polymer having a thiol end group exists inthe polymerization system when polymerization is performed. There are norestrictions as to the quantity of the polymer having a thiol end groupto be added to the polymerization system and as to the method of addingit to the polymerization system. A proper selection should be madeaccording to the desired properties of the intended block copolymer.

The radical polymerization for the block copolymer is performed in thepresence of an ordinary radical polymerization initiator such as2,2'-azobisisobutyronitrile, benzoyl peroxide, lauroyl peroxide,diisopropyl peroxycarbonate, potassium persulfate, and ammoniumpersulfate. In the case where the polymer having a thiol end group isone which is capable of polymerization in an aqueous system, likepolyethylene glycol or polyvinyl alcohol, the redox polymerization ispossible which is brought about by the thiol end group of the polymerand an oxidizing agent such as potassium bromate, potassium persulfate,ammonium persulfate, and hydrogen peroxide. Potassium bromate ispreferred for the synthesis of the block copolymer, because it does notform radicals independently under the ordinary polymerization condition,but forms radicals only when decomposed by the redox reaction with thethiol end group of the polymer.

In the production of the block copolymer, the polymerization systemshould preferably be kept acid for radical polymerization in thepresence of the polymer having a thiol end group. If the polymerizationis performed under a basic condition, the thiol group is added ionicallyto the double bond of the monomer and disappears. Polymerization in anaqueous system should preferably be performed at pH 4 or below.

Another constituent component of the block copolymer of this inventionis a homopolymer or random copolymer of the monomers capable of radicalpolymerization. There are no specific restrictions on the composition,molecular weight, and molecular weight distribution. The monomerscapable of radical (co)polymerization include vinyl acetate, vinylformate, vinyl propionete, vinyl laurate, vinyl stearate, and othervinyl ester; ethylene, propylene, isobutylene, and other olefins;acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate, dodecyl acrylate, 2-hydroxyethylacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate,2-ethylhexyl methacrylate, dodecyl methacrylate, 2-hydroxyethylmethacrylate, dimethylaminoethyl acrylate, dimethylaminoethylmethacrylate, and quaternary products thereof; acrylamide,methacrylamide, N-methylolacrylamide, N,N-dimethylacrylamide,acrylamide-2-methylpropanesulfonic acid, and sodium salts thereof, andother acrylamide monomer; styrene, α-methyl styrene, p-styrenesulfonicacid, and sodium salts and potassium salts thereof, and other aromaticvinyl monomers; acrylonitrile, methacrylonitrile, andN-vinylpyrrolidone. Also included in the monomers are vinyl chloride,vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, and othervinyl halides. These monomers are used individually or in combinationwith one another. Preferable among them are vinyl esters, acrylic acid,methacrylic acid, acrylic esters, methacrylic esters, acrylamide,methacrylamide, acrylonitrile, methacrylonitrile, vinyl halides,aromatic vinyl compounds, and α-olefins. Most suitable among them arevinyl esters, acrylic esters, methacrylic esters, acrylic acid,methacrylic acid, acrylamide, methacrylamide, and vinyl halides. In thecase where the block copolymer is to be produced, it is necessary toselect the monomers capable of radical polymerization which willconstitute the polymer component different from the polymer componenthaving a thiol end group.

The polymer having a thiol end group which is used for producing theblock copolymer should preferably be the above-mentioned polymers X, X₁,and X₂. The polymer X₂ is most suitable.

The block copolymer is composed of two components. One component is thepolymer (such as X, X₁, and X₂) having a thiol end group, and the othercomponent is a homopolymer or random copolymer of monomers capable ofradical polymerization. The polymer is not limited in its ccmposition,molecular weight, and molecular weight distribution. The ratio (byweight) of the other component to the polymer having a thiol end groupshould be 1/99 to 99/1, preferably 1/9 to 9/1, and more suitably 1/5 to5/1.

The block copolymer having as one component the polymer X₁ (vinyl esterpolymer) obtained according to this invention has properties which aredifferent from those of a polymer blend. For example, the polymerconstituting one component of the block copolymer is miscible with thepolymer constituting the other component of the block copolymer. Thusthe block copolymer will find wide use as a dispersing agent for moldingmaterials such as film and sheet, emulsifier, cement additive, binderfor inorganic materials such as ceramics, adhesive, and photosensitiveresin material.

According to one of the most preferred embodiments, the block copolymeris prepared from the polymer X₂ (PVA) having a thiol end group. Theblock copolymer is described in the following.

The block copolymer having as one component the polymer X₂ (PVA) isavailable in varied forms. The PVA polymer may be a homopolymer orcopolymer having a different degree of polymerization andsaponification, or having a diffrent composition in copolymer. The othercomponent of the block copolymer is a polymer formed from propermonomers capable of radical polymerization. The polymer may also has adifferent composition and molecular weight. For example, water solublePVA block copolymer is obtained by using as the other component of theblock copolymer a monomer providing a water soluble polymer, such asacrylic acid, acrylamide, dimethylaminoethyl methacrylate, or sodiump-styrene-sulfonate. Such a block copolymer is obtained in the form ofaqueous solution if polymerization is performed in an aqueous system,and the aqueous solution can be used as such after polymerization. Inthe case where the polymer constituting the other component of the blockcopolymer is composed of a monomer providing a water insoluble polymer,such as acrylic ester or methacrylic ester, the block copolymer isobtained in the form of emulsion if polymerization is performed in anaqueous system. The emulsion can be used as such after polymerization.

According to the process of this invention, it is possible to producethe block copolymer composed of the PVA polymer and the polymer lackingalkali resistance like the vinyl ester or acrylic ester polymer. Such ablock copolymer cannot be obtained by saponifying a block copolymerhaving a vinyl ester polymer as one component, because the othercomponent lacking alkali resistance is also hydrolyzed in thesaponification process.

The block copolymer having the PVA polymer as one component has widelyvaried properties which are different from those of a blend polymer asmentioned above. For example, compatibility of the polymer constitutingone component of the block copolymer and the polymer constituting theother component of the block copolymer is good. Thus the block copolymerwill find use in broad application areas. For example, internal size andsurface size for paper, paper coating material, textile sizing, warpsizing agent, fiber finishing agent, coating medium, coating compoundfor glass fiber, metal surface coating, defogging agent(blur-preventingagent); adhesives for wood, paper, aluminum foil, plastics and the like;binders for non-woven textile, fibers, gypsum board, fiber board and thelike; thickner for emulsion-type adhesives; additive for urea resinadhesive; binders and additives for inorganic materials such as cement,mortar, and ceramics; adhesives such as hot melt adhesive andpressure-sensitive adhesive; dispersing agent for emulsionpolymerization of ethylene, styrene, vinyl acetate, (meth)acrylic ester,vinyl chloride, vinylidene chloride, acrylonitrile, and other ethylenicunsaturated monomer, and butadiene monomer; dispersing agent andstabilizer for pigment in coating materials and adhesives; dispersingagent and stabilizer for suspension polymerization of vinyl chloride,vinylidene chloride, styrene, (meth)acrylic ester, vinyl acetate, andother ethylenic unsaturated monomers; fiber, film, sheet, pipe, tube,water-soluble fiber, temporary film, and other moldings; humectant forhydrophobic resins; additives for compound fiber and film; blending withsynthetic resin; soil conditioner and soil stabilizer; andphotosensitive resins.

A block copolymer composed of a polyvinyl alcohol component and apolymer component having amide groups or having amide groups andcarboxyl groups (e.g., polyacrylamide, polymethacrylamide,poly-N,N-dimethylacrylamide, and a copolymer of acrylamaide and acrylicacid) imparts strength and rigidity to paper when used as a papercoating material. In addition, a block copolymer composed of a polyvinylalcohol component and a polymer component having carboxyl groups(including metal salt and ammonium salt thereof) (e.g., polyacrylic acidand polymethacrylic acid), a block copolymer composed of a polyvinylalcohol component and a polymer component having cationic groups (e.g.,acrylamide propyltrimethyl ammonium chloride), and a block copolymercomposed of a polyvinyl alcohol component and a polymer component havingamide groups or having amide groups and carboxyl groups improves thetearing strength, breaking length, bursting strength, foldingresistance, and IGT pick, when used as a beater additive.

The invention is now described in detail with reference to the followingnon-limitative examples, in which quantities are expressed as parts or %by weight.

EXAMPLE 1

Into a reactor were charged 290 parts of methyl methacrylate(abbreviated as MMA hereinafter) and 4.6 parts of thiolacetic acid. Theatmosphere in the reactor was thoroughly replaced with nitrogen. Withthe external temperature of the reactor raised to 85° C., polymerizationwas carried out for 1.5 hours by adding 10 parts of MMA containing 0.3part of 2,2'-azobisisobutyronitrile. Conversion to polymer was 78.8%.The polymerization product was diluted with acetone and the solution waspoured into n-hexane to precipitate the polymer. Unreacted MMA wasremoved from the polymer by repeating three times reprecipitation withwater from an acetone solution.

200 g of this polymer was dissolved in a mixed solvent of acetone (320g) and methanol (80 g). To the solution was added 12 g of 10% NaOHsolution in methanol. Reaction was performed at 40° C. for 2.5 hours.The solution was poured into 5 liters of water containing 50 ml of 1N H₂SO₄. The polymer which had separated out and precipitated in water wasfiltered off and rinsed thoroughly with water and dried. Thus there wasobtained PMMA having a thiol group at one end. This PMMA was formed tohave an intrinsic viscosity [η] of 0.06 (dl/g) as measured in acetone at30° C. The quantity of the thiol group was 9.99×10⁻⁵ eq/g as determinedby iodimetry in acetone/water.

EXAMPLE 2

Into a reactor were charged 290 parts of n-butyl methacrylate(abbreviated as BMA hereinafter) and 3.2 parts of thiolacetic acid. Theatmosphere in the reactor was thoroughly replaced with nitrogen. Withthe internal temperature of the reactor heated to 85° C., polymerizationwas carried out for 2 hours by adding 10 parts of BMA containing 0.3part of 2,2'-azobisisobutyronitrile. Conversion to polymer was 75.2%.The polymerization product was diluted with acetone and the solution waspoured into methanol to precipitate the polymer. Unreacted BMA wasremoved from the polymer by repeating three times reprecipitation withmethanol from the acetone solution.

200 g of this polymer was dissolved in a mixed solvent of acetone (320g) and methanol (80 g). To the solution was added 10 g of 10% NaOHsolution in methanol. Reaction was performed at 40° C. for 2.5 hours.The solution was poured into 5 liters of water containing 40 ml of 1N H₂SO₄. The polymer which had separated out and precipitated in water wasfiltered off and rinsed thorroughly with water and dried. Thus there wasobtained PBMA having a thiol group at one end. This PBMA was found tohave an intrinsic viscosity [η] of 0.07 (dl/g) as measured in acetone at30° C. The quantity of the thiol group was 6.82×10⁻⁵ eq/g as determinedby iodimetry in acetone/water.

EXAMPLE 3

Into a reactor were charged 290 parts of styrene and 2.5 parts ofthiolacetic acid. The atmosphere in the reactor was thoroughly replacedwith nitrogen. With the internal temperature of the reactor heated to60° C., polymerization was carried out for 2 hours by adding 10 parts ofstyrene containing 0.3 part of 2,2'-azobisisobutyronitrile. Conversionto polymer was 15.2%. The polymerization product was poured intomethanol to precipitate the polymer. Unreacted styrene was removed fromthe polymer by repeating three times reprecipitation with methanol froman acetone solution.

35 g of this polymer was dissolved in a mixed solvent of acetone (100 g)and methanol (10 g). To the solution was added 12 g of 10% NaOH solutionin methanol. Reaction was performed at 40° C. for 2.5 hours. Thesolution was poured into 5 liters of water containing 50 ml of 1N H₂SO₄. The polymer which had separated out and precipitated in water wasfiltered off and rinsed thoroughly with water and dried. Thus there wasobtained polystyrene having a thiol end group at one end. Thispolystyrene was found to have an intrinsic viscosity [η] of 0.10 (dl/g)as measured in acetone at 30° C. The quantity of the thiol group was7.50×10⁻⁵ eq/g as determined by iodimetry in acetone/water.

EXAMPLE 4

Into a reactor were charged 290 parts of acrylic acid and 6.1 parts ofthiolacetic acid. The atmosphere in the reactor was thoroughly replacedwith nitrogen. With the internal temperature of the reactor heated to60° C., polymerization was carried out for 2 hours by adding 10 parts ofacrylic acid containing 0.3 part of 2,2'-azobisisobutyronitrile.Conversion to polymer was 35.2%. The polymerization product was pouredinto acetone to precipitate the polymer. Unreacted acrylic acid wasremoved from the polymer by repeating three times reprecipitation withmethanol from an acetone solution.

90 g of this polymer was dissolved in 100 g of methanol. To the solutionwas added 50 ml of 1N HCl in methanol. Reaction was performed at 40° C.for 5 hours. The solution was poured into acetone. The polymer which hadseparated out and precipitated in acetone was filtered off and rinsedthoroughly with acetone. The polymer was reprecipitated twice withacetone from the water and dried. Thus there was obtained polyacrylicacid having a thiol group at one end of the molecule. The sodium salt ofthe polyacrylic acid was found to have an intrinsic viscosity [η] of 0.2(dl/g) as measured in NaOH aqueous solution (2 mol/liter) at 30° C. Thequantity of the thiol group was 9.65×10⁻⁵ eq/g as determined byiodimetry in water.

EXAMPLE 5

Into an autoclave were charged 100 parts of tetrafluoroethylene, 20parts of thiolacetic acid, and 1.0 part of 2,2'-azobisisobutyronitrile.Polymerization was carried out at 80° C. for 5 hours. Unreactedthiolacetic acid was removed under reduced pressure. 100 parts ofmethanol was added under a nitrogen stream. After dissolution, 1.5 g of28% solution of sodium methylate was added. The solution was stirred atroom temperature for 1 hour. 10 ml of 1N HCl methanol solution wasadded, and then 1 liter of distilled water was added. The lower oillayer was discharged and distilled. Thus there was obtained 20 g of afraction of 40°-80° C./3 mmHg, which is polytetrafluoroethylene having athiol group at one end. The quantity of the thiol group was 3.0×10⁻³eq/g as determined by iodimetry in acetone/water.

EXAMPLE 6

Into a reactor were charged 2400 parts of vinyl acetate (abbreviated asVAc hereinafter) and 580 parts of methanol. The atmosphere in thereactor was thoroughly replaced with nitrogen. The reactor wasexternally heated to 65° C., and when the internal temperature reached60° C., 0.93 part of thiolacetic acid and 20 parts of methanolcontaining 0.868 part of 2,2'-azobisisobutyronitrile were added.Immediately thereafter, 60 parts of methanol solution containing 17.4parts of thiolacetic acid was added at a constant rate over 5 hours.Conversion to polymer was 50.4% after 5 hours. After cooling, unreactedVAc was distilled away together with methanol under reduced pressure byadding methanol to the reactor continuously. Thus there was obtained a64.5% solution of PVAc in methanol. The PVAc was saponified into PVA at40° C. in the condition so that the concentration of PVAc was 50% andthe molar ratio of NaOH to VAc became 0.05. This PVA was washed withmethanol by using a Soxhlet apparatus. The intrinsic viscosity [η] wasmeasured in water at 30° C. and the viscometric average degree ofpolymerization (P) was determined to be 130 according to the equation of[η]=7.51×10⁻³ ×P⁰.64. The degree of saponification was 98.6%.

The quantity of thiol group in the PVA as determined by iodimetry was1.87×10⁻⁴ eq/g-PVA. The number-average degree of polymerization wascalculated at 127 from the content of the thiol group. These factssuggest that the thiol group exists at only one end. This can also beinferred from the polymerization mechanism employed.

EXAMPLE 7

Into a reactor were charged 2400 parts of vinyl acetate (abbreviated asVAc hereinafter) and 580 parts of methanol. The atmosphere in thereactor was thoroughly replaced with nitrogen. The reactor wasexternally heated to 65° C., and when the internal temperature of thereactor reached 60° C., 5 parts of thiolacetic acid was added and then20 parts of methanol containing 0.868 part of2,2'-azobisisobutyronitrile. Polymerization was carried out for 2 hours.Conversion to polymer was 20.5%. After cooling, unreacted VAc wasdistilled away together with methanol under reduced pressure. This stepwas repeated by adding methanol to the reactor. Thus there was obtaineda 45.2% solution of PVAc in methanol. To a portion of this methanolsolution was added a methanol solution of NaOH so that the concentrationof PVAc became 30% and the molar ratio of NaOH to VAc became 0.05. ThePVAc was saponified into PVA at 40° C. This PVA was washed with methanolby using a Soxhlet apparatus. The degree of polymerization measured inthe same way as in Example 6 was 90, and the degree of saponificationwas 99.0 mol %.

The quantity of thiol group in the PVA as determined by iodimetry was2.90×10⁻⁴ eq/g-PVA. The presence of the terminal thiol group wasconfirmed.

EXAMPLES 8 to 10

Polymerization was carried out as in Example 6, with the quantity ofthiolacetic acid varied, and the resulting polymer was saponified togive PVA. Table 1 shows the conditions of polymerization and thequantity of thiol group in the product.

                                      TABLE 1                                     __________________________________________________________________________    Quantity of    Conversion                                                                          PVA                                                           thiolacetic acid                                                                        to polymer                                                                          Degree of      Quantity of                               Example                                                                            added (parts)                                                                           after 5                                                                             saponification                                                                       Degree of                                                                             thiol group                               No.  Initial                                                                           Subsequent                                                                          hr (%)                                                                              (mol %)                                                                              polymerization                                                                        (eq/g-PVA)                                __________________________________________________________________________    8    0.45                                                                               8.41 65.7  98.7   250     1.30 × 10.sup.-4                    9    0.29                                                                              5.4   66.0  99.0   412     9.08 × 10.sup.-5                    10   0.16                                                                              3.0   68.2  98.5   567     5.44 × 10.sup.-5                    __________________________________________________________________________

EXAMPLE 11

Into a reactor were charged 2400 parts of vinyl formate and 580 parts ofmethanol. The atmosphere in the reactor was thoroughly replaced withnitrogen. The reactor was heated externally to 65° C., and when theinternal temperature reached 60° C., 6.3 parts of thiolacetic acid wasadded and then 20 parts of methanol containing 0.868 part of2,2'-azobisisobutyronitrile. Polymerization was carried out for 2 hours.Conversion to polymer was 19.7%. After cooling, unreacted vinyl formatewas distilled away together with methanol under reduced pressure. Thisstep was repeated by adding methanol to the reactor. Thus there wasobtained a 39.8% solution of polyvinyl formate in methanol. To a portionof this methanol solution was added a methanol solution of NaOH so thatthe concentration of polyvinyl formate became 30% and the molar ratio ofNaOH to vinyl formate became 0.05. The polyvinyl formate was saponifiedinto PVA at 40° C. This PVA was washed with methanol by using a Soxhletapparatus. The degree of polymerization measured in the same way as inExample 6 was 92, and the degree of saponification was 98.7 mol %.

The quantity of thiol group in the PVA as determined by iodimetry was3.05×10⁻⁴ eq/g-PVA. The presence of the terminal thiol group wasconfirmed.

EXAMPLE 12

Into a reactor were charged 2400 parts of vinyl acetate (abbreviated asVAc hereinafter) and 580 parts of methanol. The atmosphere in thereactor was thoroughly replaced with nitrogen. The reactor wasexternally heated to 65° C., and when the internal temperature reached60° C., 1.10 parts of thiolpropionic acid and 20 parts of methanolcontaining 0.868 part of 2,2'-azobisisobutyronitrile were added.Immediately thereafter, 60 parts of methanol solution containing 20.6parts of thiolpropionic acid was added at a constant rate over 5 hours.The rate of polymerization was 49.6%. After cooling, unreacted VAc wasdistilled away together with methanol under reduced pressure. This stepwas repeated by adding methanol to the reactor. Thus there was obtaineda 65.3% solution of PVAc in methanol. To a portion of this methanolsolution was added a methanol solution of NaOH so that the concentrationof PVAc became 50% and the molar ratio of NaOH to VAc became 0.05. ThePVAc was saponified into PVA at 40° C. This PVA was washed with methanolby using a Soxhlet apparatus. The degree of polymerization measured inthe same way as in Example 6 was 120, and the degree of saponificationwas 99.0 mol %.

The quantity of thiol group in the PVA as determined by iodimetry was1.85×10⁻⁴ eq/g-PVA. The presence of the terminal thiol group wasconfirmed.

EXAMPLE 13

Into a reactor were charged 2400 parts of vinyl acetate (abbreviated asVAc hereinafter) and 580 parts of methanol. The atmosphere in thereactor was thoroughly replaced with nitrogen. The reactor wasexternally heated to 65° C., and when the internal temperature reached60° C., 0.93 part of thiolacetic acid and 20 parts of methanolcontaining 0.868 part of 2,2'-azobisisobutyronitrile were added.Immediately thereafter, 60 parts of methanol solution containing 17.4parts of thiolacetic acid was added at a constant rate over 5 hours.Conversion to polymer was 50.4%. After cooling, unreacted VAc wasdistilled away together with methanol under reduced pressure. This stepwas repeated by adding methanol to the reactor. Thus there was obtaineda 64.5% solution of PVAc in methanol.

To 31 g of this methanol solution was added 5 ml of 25% ammonia waterwith stirring at 25° C. for 10 minutes. The solution was poured intowater to precipitate the polymer. The polymer was purified byreprecipitation with water from an acetone solution. The intrinsicviscosity [η] measured in acetone at 30° C. was used to obtain theviscometric average degree of polymerization according to the equationof [η]=7.94×10⁻³ ×P⁰.64. The viscometric average degree ofpolymerization was 125. The content of vinyl alcohol units calculatedfrom the degree of saponification was 10.2 mol %.

The iodimetry in methanol/water (80/20 by weight) indicated the presenceof thiol groups in an amount of 1.41×10⁻⁴ eq/g-PVAc. The number-averagedegree of polymerization was calculated at 82 from the content of thethiol group. These facts suggest that the thiol group exists at only oneend. This can also be inferred from the polymerization mechanismemployed.

EXAMPLE 14

Into a reactor were charged 2400 parts of VAc and 580 parts of methanol.The atmosphere in the reactor was thoroughly replaced with nitrogen. Thereactor was externally heated to 65° C., and when the internaltemperature of the reactor reached 60° C., 5 parts of thiolacetic acidwas added, and then 20 parts of methanol containing 0.868 part of2,2'-azobisisobutyronitrile was dded. Polymerization was performed for 2hours. Conversion to polymer was 20.5%. After cooling, unreacted VAc wasdistilled away together with methanol under reduced pressure. This stepwas repeated by adding methanol to the reactor. Thus there was obtaineda 45.2% solution of PVAc in methanol.

To 44.3 g of this methanol solution was added 5 ml of 25% ammonia waterwith stirring at 25° C. for 10 minutes. The solution was poured intowater to precipitate the polymer. The polymer was purified byreprecipitation with water from an acetone solution.

The degree of polymerization as measured in the same way as in Example13 was 92, and the content of vinyl alcohol was 9.8 mol %. The iodimetryin methanol/water (80/20 by weight) indicated the presence of thiol endgroup in an amount of 1.50×10⁻⁴ eq/g-PVAc.

EXAMPLES 15 to 17

Polymerization was carried out as in Example 13, with the quantity ofthiolacetic acid varied, to give PVAc. The PVAc was treated with 25%ammonia water to give PVAc having thiol end groups. The results areshown Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Quantity of    Conversion                                                                          Concentration                                                                         Ammonia treatment            Content                  thiolacetic acid                                                                        to polymer                                                                          of PVAc PVAc  25%       Quantity of                                                                          Degree                                                                              of vinyl            Example                                                                            added (parts)                                                                           after 5                                                                             methanol                                                                              methanol                                                                            ammonia                                                                            Reaction                                                                           thiol group                                                                          polymeri-                                                                           alcohol             No.  Initial                                                                           Subsequent                                                                          hr (%)                                                                              solution (%)                                                                          solution (g)                                                                        water                                                                              time (eq/g-PVAc)                                                                          zation                                                                              (mol                __________________________________________________________________________                                                              %)                  15   0.45                                                                               8.41 65.7  60.5    33.1  5 ml 10 min                                                                             7.10 × 10.sup.-5                                                               248   9.6                 16   0.29                                                                              5.4   66.0  53.2    37.6  3 ml 10 min                                                                             4.60 × 10.sup.-5                                                               405   7.2                 17   0.16                                                                              3.0   68.2  48.5    41.3  3 ml 10 min                                                                             2.91 × 10.sup.-5                                                               510   5.8                 __________________________________________________________________________

EXAMPLE 18

Into a reactor were charged 1920 parts of vinyl acetate (abbreviated asVAc hereinafter), 480 parts of vinyl ester of Versatic acid ("Veova 10",a product of Shell Chemical) and 580 parts of methanol. The atmospherein the reactor was thoroughly replaced with nitrogen. The reactor wasexternally heated to 65° C., and when the internal temperature reached60° C., 0.50 part of thiolacetic acid and 20 parts of methanolcontaining 0.868 part of 2,2'-azobisisobutyronitrile were added.Immediately thereafter, 60 parts of methanol solution containing 11.5parts of thiolacetic acid was added at a constant rate over 5 hours.Polymerization was performed for 5 hours. Conversion to polymer was49.6%. After cooling, unreacted VAc was distilled away together withmethanol under reduced pressure. This step was repeated by addingmethanol to the reactor. Thus there was obtained a 47.2% methanolsolution of a copolymer composed of vinyl acetate and vinyl ester ofVersatic acid.

To 42.4 g of this methanol solution was added 3 ml of 50% dimethylaminesolution with stirring at 25° C. for 10 minutes. The solution was pouredinto water to precipitate the polymer. The polymer was purified byreprecipitation twice with hexane from an acetone solution. Theintrinsic viscosity [η] of the copolymer as measured in acetone at 30°C. was 0.14. The content of vinyl alcohol calculated from the degree ofsaponification was 9.8 mol %. The iodimetry in methanol/water (80/20 byweight) indicated the presence of thiol end groups in an amount of0.98×10⁻⁴ eq/g-polymer.

In the meantime, the polymers having thiol end groups which wereobtained in Examples 1 to 18 were heated in the air at 70° C. for 24hours. The heated polymers were completely soluble in the solvent usedfor the iodimetry for the determination of thiol groups. No change wasobserved in the content of thiol groups in the polymers before and afterheating. This indicates that the polymers are not oxidized in the airand do not form the disulfide linkage of thiol groups.

EXAMPLE 19

Polymer No. 1. Polyethylene glycol having two thiol groups at both endswas prepared as follows:

500 g of polyethylene glycol (average molecular weight: 3000, a productof Nippon Oils and Fats Co., Ltd.) was dissolved in a mixed solvent ofmethylene chloride (200 ml) and pyridine (80 g). To the solution wasadded 200 g of tosyl chloride. Reaction was performed at 25° C. for 12hours. The resulting salt was filtered off and the filtrate was pouredinto a large excess of ether so that polyethylene glycol ditosylateseparated out. The product was filtered off, washed thoroughly withether, and dried. 500 g of this polyethylene glycol ditosylate wasdissolved in 300 ml of ethanol. To this solution was added 50 g ofthiourea, and reaction was performed under reflux for 96 hours. Then, 27g of sodium hydroxide and 300 g of distilled water were added under anitrogen stream, and reaction was performed under reflux for 3 hours.After cooling to room temperature, the solution was neutralized to pH 3to 3.5 with 10% aqueous solution of H₂ SO₄. The reaction product wasextracted with 1 liter of methylene chloride. Thus there was obtained asolution of α,ω-dithiolpolyoxyethylene in methylene chloride. Thissolution was then poured into ether to precipitateα,ω-dithiolpolyoxyethylene. This product was reprecipitated twice withmethanol from the ether solution. The content of thiol groups in thisproduct as determiend by iodimetry was 5.64×10⁻⁴ eq/g.

Polymer No. 2. (Polymethyl methacrylate having a thiol end group asobtained in Example 1)

Ten parts of No. 1 polymer was dissolved in 110 parts of distilledwater, and the solution was adjusted to pH 3 with 0.5N H₂ SO₄. To thesolution was added 10 parts of N,N-dimethylacrylamide and then 10 partsof an aqueous solution containing 0.94 part of potassium bromate.Polymerization was performed at 30° C. for 2 hours. Conversion topolymer was 100.1%. There was obtained a 14.4% aqueous solution of blockcopolymer of polyethylene glycol and poly(N,N-dimethylacrylamide).

This block copolymer was made into film by casting a 10% aqueoussolution. On the other hand, a 10% aqueous solution of polymer blendhaving the same composition as the block copolymer was also made intofilm by casting. (This polymer blend is a mixture of polyethylene glycoland poly(N,N-dimethylacrylamide.) The film made of the block copolymerwas clear and uniform, whereas the film made of the mixture was cloudywith phase separation.

EXAMPLE 20

Ten parts of No. 1 polymer (prepared in Example 19) was dissolved in 110parts of distilled water, and the solution was adjusted to pH 3 with0.5N H₂ SO₄. To the solution was added 10 parts of acrylamide. Theatmosphere was replaced with nitrogen, and the reactor was heated to 60°C. Polymerization was started by adding an aqueous solution containing0.94 g of potassium bromate at a constant rate of 2 ml/5 min over 30minutes. The polymerization came to an end in 90 minutes. The conversionto polymer was 99.3%. Thus there was obtained a 14.2% (solidconcentration) aqueous solution of PEG-polyacrylamide block copolymer. Aclear film was obtained from this aqueous solution by casting.

EXAMPLE 21

Polymerization was carried out under the same conditions as in Example20, except that acrylamide was replaced by methyl acrylate. Thepolymerization came to an end in 2 hours. Conversion to polymer was98.7%. Thus there was obtained a 14.1% (solid concentration) aqueousdispersion of PEG-polymethyl acrylate block copolymer. A clear film wasobtained from this aqueous dispersion by casting.

EXAMPLE 22

0.1 part of PVA 217 (polyvinyl alcohol, degree of polymerization: 1700,degree of saponification: 88 mol %, Product of Kuraray Co., Ltd.) wasdissolved in 100 parts of water at 90° C. The solution was cooled to 60°C. under a nitrogen stream. To this solution was added a styrenesolution composed of 100 parts of styrene, 50 parts of No. 2 polymer (asobtained in Example 1), and 0.2 part of 2,2'-azobisisobutyronitrile.Suspension polymerization was performed at 60° C. for 20 hours.Conversion to polymer was 98.2%. After filtration, rinsing, and drying,there was obtained 145 parts of polymethyl methacrylate-polystyreneblock copolymer. A clear film was obtained by casting from a toluenesolution of this block copolymer.

EXAMPLE 23

0.05 part of PVA 217 (polyvinyl alcohol, degree of polymerization: 1700,degree of saponification: 88 mol %, Product of Kuraray Co., Ltd.) wasdissolved in 100 parts of water at 90° C. The solution was cooled to 60°C. under a nitrogen stream. To this solution was added a vinyl acetatesolution composed of 100 parts of vinyl acetate, 30 parts of No. 2polymer (as obtained in Example 1), and 0.1 part of benzoyl peroxide.Suspension polymerization was performed at 60° C. for 10 hours.Conversion to polymer was 92%. Unreacted monomer was removed underreduced pressure. After filtration, rinsing, and drying, there wasobtained 140 parts of polymethyl methacrylate-polyvinyl acetate blockcopolymer. 50 parts of this block copolymer was dissolved in 50 parts ofmethanol, and to the solution was added at 40° C. an NaOH solution inmethanol so that the molar ratio of NaOH to vinyl acetate became 0.05.Thus there was obtained a polymethyl methacrylate-polyvinyl alcoholblock copolymer. A clear film was obtained by casting methanol solutionof polymethyl methacrylate-polyvinyl acetate block copolymer.

EXAMPLE 24

Preparation of polyvinyl acelate-polyoxyethylene block copolymer andpolyvinyl alcohol-polyoxyethylene block copolymer.

Into a reactor were charged 960 parts of vinyl acetate (abbreviated asVAc hereinafter), 220 parts of methanol, and 4.20 parts ofα,ω-dithiolpolyoxyethylene 4000. The atmosphere in the reactor wasthoroughly replaced with nitrogen. The reactor was externally heated to65° C., and when the internal temperature reached 60° C., 20 parts ofmethanol containing 0.694 part of 2,2'-azobisisobutyronitrile was added.Immediately thereafter, 90 parts of methanol solution containing 49.9parts of No. 1 polymer obtained in Example 19(α,ω-dithiolpolyoxyethylene 4000) was added at a constant rate over 4.5hours. Conversion to polymer for 4.5 hours was 50.6%. After cooling,unreacted VAc was distilled away together with methanol under reducedpressure. This step was repeated by adding methanol.

Thus there was obtained a 43.2% solution of polyvinylacetate-polyoxyethylene block copolymer in methanol. To this methanolsolution was added at 40° C. an NaOH solution in methanol so that themolar ratio of NaOH to vinyl acetate became 0.05. The solution wasallowed to stand for saponification. Thus there was obtained a blockcopolymer of polyvinyl alcohol and polyoxyethylene. The block copolymerwas washed with methanol, purified, and dried to give white powder. Theintrinsic viscosity of polyvinyl acetate-polyoxyethylene block copolymerwas 0.52 dl/g as measured in acetone at 30° C. {[η]=η_(sp/c) /(1+0.275η_(sp)) according to single-point determination.} The weightfraction of polyoxyethylene in the block copolymer was found to be 11.2%by H-NMR (in DMSO-d₆). The degree of saponification of the PVA portionwas 99.0 mol %. Clear films were obtained by casting a methanol solutionof polyvinyl acetate-polyoxyethylene block copolymer and by casting awater solution of polyvinyl alcohol-polyoxyethylene block copolymer.

EXAMPLES 25 to 27

Polyvinyl acetate-polyoxyethylene block copolymers and polyvinylalcohol-polyoxyethylene block copolymers, both with the quantities ofα,ω-dithiolpolyoxyethylene 4000 (No. 1 polymer prepared in Example 19)varied, were prepared in the same way as in Example 24. Table 3 showsthe polymerization conditions employed, the results of polymerizaiton,and the properties of the resulting block copolymers. A 10% aqueoussolution of polyvinyl alcohol-polyoxyethylene block copolymer obtainedin Example 27 was made into film by casting. On the other hand, a 10%aqueous solution of polymer blend having the same composition as thisblock copolymer was also made into film by casting. (This polymer blendis a mixture of polyvinyl alcohol and polyoxyethylene.)

                                      TABLE 3                                     __________________________________________________________________________              Q'ty of α,ω-dithiolpoly-                                Quantity  oxyethylene 4000 (g)                                                                      Polymer-                                                                           Conversion                                                                              Content of                                                                             Degree of                            of AIBN                                                                            Initial                                                                            Subsequent                                                                           ization                                                                            to polymer                                                                          [η]                                                                           polyoxyethylene                                                                        saponification of               Example                                                                            (g)  charge                                                                             charge time (hr)                                                                          (%)   (dl/g)                                                                            (wt %)   PVA (mol %)                     __________________________________________________________________________    25   0.173                                                                              0.60 7.12   4.5  38.7  0.78                                                                              4.5      98.9                            26   0.347                                                                              0.60 7.12   4.5  42.5  0.68                                                                              7.0      98.7                            27   0.347                                                                              4.20 49.9   4.5  26.2  0.32                                                                              21.7     99.0                            __________________________________________________________________________

The film made of the block copolymer was clear and uniform, whereas thefilm made of the mixture was cloudy with phase separation.

EXAMPLE 28

α,ω-dithiolpolyoxyethylene 6000 was prepared in the same way as inExample 19 for No. 1 polymer, except that polyethylene glycol 4000 wasreplaced by polyethlene glycol 6000 (average molecular weight: 8500, aproduct of Wako Yakuhin Co., Ltd.) The quantity of thiol groupsdetermined by iodimetry was 1.67×10⁻⁴ eq/g. Using thisα,ω-dithiolpolyoxyethylene 6000, polymerization was performed in thesame way as in Example 24. There were obtained a polyvinylacetate-polyoxyethylene block copolymer and a polyvinylalcohol-polyoxyethylene block copolymer. Table 4 shows thepolymerization conditions employed, the results of polymerizaiton, andthe properties of the resulting block copolymers.

A 10% aqueous solution of the polyvinyl alcohol-polyoxyethylene blockcopolymer obtained in Example 28 was made into film by casting. On theother hand, a 10% aqueous solution of polymer blend having the samecomposition as this block copolymer was also made into film by casting.(This polymer blend is a mixture of polyvinyl alcohol andpolyoxyethylene.) The film made of the block copolymer was clear anduniform, whereas the film made of the mixture was cloudy with phaseseparation.

                                      TABLE 4                                     __________________________________________________________________________              Q'ty of α,ω-dithiolpoly-                                Quantity  oxyethylene 6000 (g)                                                                      Polymer-                                                                           Conversion                                                                              Content of                                                                             Degree of                            of AIBN                                                                            Initial                                                                            Subsequent                                                                           ization                                                                            to polymer                                                                          [η]                                                                           polyoxyethylene                                                                        saponification of               Example                                                                            (g)  charge                                                                             charge time (hr)                                                                          (%)   (dl/g)                                                                            (wt %)   PVA (mol %)                     __________________________________________________________________________    28   0.173                                                                              5.07 66.9   5.0  13.6  0.46                                                                              34.3     98.5                            __________________________________________________________________________

EXAMPLES 29 to 30

Preparation of polypropylene glycol having thiol groups at both ends.

200 g of polypropylene glycol 2000 (average molecular weight: 2000, aproduct of Sanyo Chemical Co., Ltd.) was dissolved in 200 ml ofpyridine. To the solution was added 77 g of tosyl chloride, and reactionwas performed at 25° C. for 24 hours. The resulting salt was filteredoff and the filtrate was poured into a large amount of distilled water.Polypropylene glycol ditosylate thus obtained was separated andthoroughly washed with distilled water and dried. 200 g of thispolypropylene glycol ditosylate was dissolved in 300 ml of ethanol. Tothe solution was added 30 g of thiourea, and reaction was performed for120 hours under reflux. Then, 17 g of sodium hydroxide and 100 g ofdistilled water were added under a nitrogen stream, and reaction wasperformed for 3 hours under reflux. After cooling to room temperature,the solution was neutralized to pH 3 to 3.5 with a 10% H₂ SO₄ aqueoussolution. By extraction with 1 liter of methylene chloride, there wasobtained a methylene chloride solution of α,ω-dithiolpolyoxypropylene.The methylene chloride solution was thoroughly washed with water anddehydrated with Na₂ SO₄. Methylene chloride was distilled away underreduced pressure. Thus there was obtained α,ω-dithiolpolyoxypropylene.The quantity of thiol groups in this product determined by iodimetry was4.80×10⁻⁴ eq/g.

Preparation of block copolymer.

Using this α,ω-dithiolpolyoxyethylene 2000, polymerization was performedin the same way as in Example 24. There were obtained a polyvinylacetate-polyoxypropylene block copolymer and a polyvinylalcohol-polyoxypropylene block copolymer. Table 5 shows thepolymerization conditions employed, the results of polymerization, andthe properties of the resulting block copolymers.

Clear films were obtained by casting aqueous solutions of polyvinylalcohol-polyoxypropylene block copolymers.

EXAMPLES 31 and 32

Preparation of polyvinyl acetate-polymethyl methacrylate block copolymerand polyvinyl alcohol-polymethyl methacrylate block copolymer.

A block copolymer was obtained in the same way as in Example 24 frompolymethyl metharcylate having a thiol group at one end which wasobtained in Example 1, except that methanol as a solvent forα-thiol-PMMA was replaced by methyl acetate.

                                      TABLE 5                                     __________________________________________________________________________              Q'ty of α,ω-dithiolpoly-                                Quantity  oxyethylene 2000 (g)                                                                      Polymer-                                                                           Conversion                                                                              Content of                                                                             Degree of                            of AIBN                                                                            Initial                                                                            Subsequent                                                                           ization                                                                            to polymer                                                                          [η]                                                                           polyoxypropylene                                                                       saponification of               Example                                                                            (g)  charge                                                                             charge time (hr)                                                                          (%)   (dl/g)                                                                            (wt %)   PVA (mol %)                     __________________________________________________________________________    29   0.173                                                                              0.71 5.58   3.0  29.3  0.67                                                                               1.9     98.7                            30   0.173                                                                              1.76 18.6   4.0  26.1  0.40                                                                              12.0     98.5                            __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Quantity  Q'ty of -thiol- PMMA (g)                                                                   Polymer-                                                                           Conversion        Degree of                            of AIBN                                                                            Initial                                                                            Subsequent                                                                            ization                                                                            to polymer                                                                          [η]*                                                                          Content of                                                                            saponification of               Example                                                                            (g)  charge                                                                             charge  time (hr)                                                                          (%)   (dl/g)                                                                            PMMA (wt %)                                                                           PVA (mol %)                     __________________________________________________________________________    31   0.173                                                                              1.69 15.6    3.0  37.8  0.65                                                                               7.5    99.0                            32   0.173                                                                              4.65 42.9    3.0  36.8  0.45                                                                              18.9    98.6                            __________________________________________________________________________     *Polyvinyl acetatepolymethyl methacrylate block copolymer: measured in        acetone at 30° C.                                                 

Table 6 shows the polymerization conditions employed, the results ofpolymerization, and the properties of the resulting block copolymers.

Clear films were obtained by casting aqueous solutions of polyvinylalcohol-polymethyl methacrylate block copolymers.

EXAMPLE 33

No. 3 polymer. (Vinyl ester polymer having a thiol group at one end asobtained in Example 13) No. 4 polymer and No. 5 polymer.

Two kinds of PVAc were prepared in the same way as for No. 3 polymer,with the quantity of thiolacetic acid varied. They were treated with 25%ammonia water to give PVAc having a thiol group. The results are shownin Table 7.

                  TABLE 7                                                         ______________________________________                                        Thiolacetic acid                                                                            subse-  Degree       Content                                                                              Content                                   Initial quent   of pol-                                                                             Content                                                                              of vinyl                                                                             of vinyl                            PVAc  charge  charge  ymeri-                                                                              of SH  alcohol                                                                              acetate                             No.   (parts) (parts) zation                                                                              (eq/g) (mol %)                                                                              (mol %)                             ______________________________________                                        4     0.45    8.41    248   1.30 ×                                                                         9.6    90.4                                                            10.sup.-4                                         5     0.29    5.40    405   9.08 ×                                                                         7.2    92.8                                                            10.sup.-5                                         ______________________________________                                    

50 parts of vinyl ester polymer (No. 3 polymer) was dissolved in 280parts of toluene. To the solution was added 100 parts of methylmethacrylate (abbreviated as MMA). The atmosphere in the reactor wasreplaced with nitrogen, and the reactor was heated. When the internaltemperature reached 80° C., 20 parts of toluene containing 0.3 part of2,2'-azobisisobutyronitrile was added. After polymerization for 5 hours,there was obtained a toluene solution of polyvinyl ester-PMMA blockcopolymer. The solid concentration was 32.7%.

A 5% acetone solution of the polyvinyl ester-PMMA block copolymerobtained in Example 33 was made into film by casting. On the other hand,a 5% acetone solution of a polymer blend having the same composition asthis block copolymer was also made into film by casting. (This polymerblend is a mixture of polyvinyl ester and PMMA.) The film made of theblock copolymer was clear and uniform, whereas the film made of thepolymer blend was cloudy with phase separation.

EXAMPLES 34 to 38

Five kinds of block copolymers, each containing a vinyl ester polymer asone component, were prepared in the same way as in Example 33 from vinylester polymers (No. 3 to No. 5 polymers). The results are shown in Table8. (The polymerization temperature was 60° C. in the case where methanolwas used as the solvent for polymerization.)

EXAMPLE 39

0.1 part of PVA 217 (polyvinyl alcohol, degree of polymerization: 1700,degree of saponification: 88 mol %, Product of Kuraray Co., Ltd.) wasdissolved in 100 parts of distilled water at 90° C. The solution wascooled to 60° C. under a nitrogen stream. To this solution was added astyrene solution composed of 100 parts of styrene, 50 parts of No. 4polymer, and 0.3 part of 2,2'-azobisisobutyronitrile. Suspensionpolymerization was performed at 60° C. for 20 hours. Conversion topolymer was 99.0%.

After filtration, rinsing, and drying, there was obtained 144 parts ofpolyvinyl ester-polystyrene block copolymer. A clear film was obtainedby casting a toluene solution of this block copolymer.

                                      TABLE 8                                     __________________________________________________________________________                                          Polymerization                                                                        Film obtained                               Polyvinyl ester                                                                       Monomer           Time                                                                              Solids                                                                            from polymer                    Example                                                                            Solvent                                                                              No.                                                                              Quantity                                                                           Kind         Quantity                                                                           (hr)                                                                              (%) Solvent                                                                            Film                       __________________________________________________________________________    34   Methanol 300                                                                         4  50   Acrylic acid 100  5.0 33.5                                                                              Methanol                                                                           Clear                      35   Toluene 300                                                                          3  30   Styrene      100  20.0                                                                              30.0                                                                              Toluene                                                                            Clear                      36   Toluene 300                                                                          5  20   n-Butyl acrylate                                                                           100  5.0 27.9                                                                              Toluene                                                                            Clear                      37   Methanol 300                                                                         4  50   N,N--dimethyl-acrylamide                                                                   100  5.0 33.6                                                                              Methanol                                                                           Clear                      38   Methanol 300                                                                         3  20   Methacrylic acid                                                                           100  5.0 28.7                                                                              Methanol                                                                           Clear                      __________________________________________________________________________

EXAMPLE 40

No. 6 polymer (PVA polymer having a thiol group at one end as preparedin Example 6)

No. 7 polymer and No. 8 polymer.

Two kinds of PVA polymers having a thiol group at one end were preparedby polymerizing and saponifying in the same way as for No. 6 polymer,with the quantity of thiolacetic acid varied. The results are shown inTable 9.

                  TABLE 9                                                         ______________________________________                                        Thiolacetic acid                                                                     subse-                                                                              Properties of PVA                                                     Initial quent   Degree of                                                                              Content  Degree of                              PVA  charge  charge  saponification                                                                         of SH    polymer-                               No.  (parts) (parts) (mol %)  (eq/g)   ization                                ______________________________________                                        7    0.45    8.41    98.7     1.30 × 10.sup.-4                                                                 250                                    8    0.29    5.40    99.0     9.08 × 10.sup.-5                                                                 412                                    ______________________________________                                         Note: (Concentration of PVAc at the time of saponification was 40%.)     

No. 9 polymer.

A portion of the methanol solution of PVAc obtained in the process ofpreparing No. 7 polymer was saponified at 40° C. under the followingconditions to give a partially saponified product.

Concentration of PVAc: 40%.

Molar ratio of NaOH to VAc: 0.010.

The results are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                               Degree of                                                                     saponification                                                                             Content of Degree of                                      PVA No.                                                                              (mol %)      SH (eq/g)  polymerization                                 ______________________________________                                        9      86.5         1.35 × 10.sup.-4                                                                   240                                            ______________________________________                                    

Ten parts of No. 6 polymer (PVA having a thiol group at one end) wasdissolved in 110 parts of distilled water at 95° C. The solution wascooled to 30° C. under a nitrogen stream. To the solution were added 10parts of acrylic acid and then 10 parts of aqueous solution containing0.32 part of potassium bromate. Polymerization was started at 30° C. andcontinued for 2 hours. Conversion to polymer was 100.1%. There wasobtained a 14.4% (solid concentration) aqueous solution ofPVA-polyacrylic acid block copolymer. A clear film was obtained fromthis aqueous solution by casting.

EXAMPLE 41

Ten parts of No. 8 polymer (PVA having a thiol group at one end) wasdissolved in 108 parts of distilled water at 95° C. The solution wascooled to room temperature under a nitrogen stream. To the solution wasadded 10 parts of acrylic acid. The temperature was raised to 60° C.,and polymerization was started by adding 12 parts of aqueous solutioncontaining 0.152 part of potassium bromate at a constant rate of 2 ml/5min over 30 minutes. Polymerization was continued for 2 hours.Conversion to polymer was 101.4%.

There was obtained a 14.9% (solid concentration) aqueous solution ofPVA-polyacrylic acid block copolymer. A clear film was obtained fromthis aqueous solution by casting.

EXAMPLE 42

Ten parts of No. 7 polymer (PVA having a thiol group at one end) wasdissolved in 108 parts of distilled water at 95° C. The solution wascooled to room temperature. The solution was adjusted to pH 3 with 0.5NH₂ SO₄. To the solution was added 10 parts of acrylamide. The atmospherein the reactor was replaced with nitrogen and the temperature was raisedto 60° C. Polymerization was started by adding an aqueous solutioncontaining 0.217 part of potassium bromate at a contant rate of 2 ml/5min over 30 minutes. Polymerization was continued for 90 minutes.

Conversion to polymer was 101.7%. There was obtained a 14.5% (solidconcentration) aqueous solution of PVA-polyacrylamide block copolymer.

A 5% aqueous solution of the PVA-polyacrylamide block copolymer obtainedin Example 42 was made into film by casting. On the other hand, a 5%aqueous solution of a polymer blend having the same composition as thisblock copolymer was also made into film by casting. (This polymer blendis a mixture of PVA and polyacrylamide.) The film made of the blockcopolymer was clear and uniform, whereas the film made of the polymerblend was cloudy with phase separation.

EXAMPLES 43 to 46

Block copolymers having PVA as one component were prepared in the sameway as in Example 42. Table 11 shows the conditions and results ofpolymerization.

The aqueous emulsions of polymers obtained in Examples 44 to 46 weremade into film by casting.

The resulting films were clear. On the other hand, emulsions of polymerblends each having the same composition as the polymers obtained inExamples 44 and 46 were made into films by casting.

                                      TABLE 11                                    __________________________________________________________________________                              KBrO.sub.3                                                                             Results of Polymerization                           PVA     Monomer  Quan-                                                                             Time for                                                                           Polymer-                                                                            Conversion                                                                          Solid                          Example                                                                            Water                                                                             No.                                                                              Quanity                                                                            Kind                                                                              Quantity                                                                           tity                                                                              addition                                                                           ization time                                                                        to polymer                                                                          content                        __________________________________________________________________________    43   120 7  10   DMA 10   0.217                                                                             60 min                                                                               1.5 hr                                                                             99.3%                                                                               14.1%                         44   120 7  10   EA  9.5  0.217                                                                             60   1.5   96.8  13.9                                            AA  0.5                                                      45   120 8  10   VAc 10   0.152                                                                             60   2.0   100.2 14.3                           46   120 6  10   St  10   0.320                                                                             60   2.0   98.5  14.0                           __________________________________________________________________________     DMA: N,N--dimethylacrylamide                                                  EA: Ethyl acrylate                                                            AA: Acrylic acid                                                              VAc: Vinyl acetate                                                            St: Styrene                                                              

These polymer blends are mixtures of EA-AA copolymer emulsion and PVA,and polystyrene emulsion and PVA.) The films made of the polymer blendswere hazy. A partially saponified polyvinyl alcohol (degree ofsaponification 49.5 mol %) was dissolved in a mixed solvent ofmethanol-water (50/50). This solution was made into film by casting.

The resulting film had a tensile strength of 1 kg/mm². Also, an aqueousemulsion of the polymer obtained in Example 45 was made into film bycasting. The resulting film was clear and had a tensile strength of 4kg/mm².

A 5% aqueous solution of the PVA-N,N-dimethylacrylamide block copolymerobtained in Example 43 was made into film by casting. On the other hand,a 5% aqueous solution of a polymer blend having the same composition asthis block copolymer was also made into film by casting. (This polymerblend is a mixture of PVA and poly-N,N-dimethylacrylamide.) The filmmade of the block copolymer was clear and uniform, whereas the film madeof the polymer blend was cloudy with phase separation.

EXAMPLE 47

Ten parts of No. 8 polymer (PVA having a thiol group at one end) wasdissolved in 110 parts of distilled water at 95° C. The solution wascooled to room temperature. The solution was adjusted to pH 3 with 0.5NH₂ SO₄. To the solution was added 10 parts of acrylamide. The atmospherein the reactor was replaced with nitrogen and the temperature was raisedto 60° C. Polymerization was started by adding all at once an aqueoussolution containing 0.2 part of potassium persulfate. Polymerizationtook 2 hours.

Conversion to polymer was 99.7%. There was obtained a 14.3% (solidconcentration) aqueous solution of PVA-polyacrylamide block copolymer.

A 5% aqueous solution of the PVA-polyacrylamide block copolymer obtainedin Example 47 was made into film by casting. On the other hand, a 5%aqueous solution of a polymer blend having the same composition as thisblock copolymer was also made into film by casting. (This polymer blendis a mixture of PVA and polyacrylamide.) The film made of the blockcopolymer was clear and uniform, whereas the film made of the polymerblend was cloudy with phase separation.

EXAMPLE 48

Polymerization was performed in the same way as in Example 47, exceptthat the monomer was replaced by methyl acrylate. Polymerization took 2hours. Conversion to polymer was 99.6%. There was obtained a 14.3%(solid concentration) aqueous dispersion of PVA-polymethyl acrylateblock copolymer.

A clear film was obtained from this aqueous dispersion by casting.

EXAMPLE 49

Ten parts of 30% aqueous solution of No. 6 polymer (PVA having a thiolgroup at one end) was dissolved in 43 parts of distilled water. Thesolution was adjusted to about pH 3 with 0.5N H₂ SO₄. To the solutionwere added 40 parts of methanol and 17 parts of methyl methacrylate. Theatmosphere in the reactor was replaced with nitrogen and the temperaturewas raised to 65° C. Polymerization was started by adding 10 parts ofmethanol solution containing 0.034 parts of 2,2'-azobisisobutyronitrile.Conversion to polymer after 6 hours of polymerization was 98%. There wasobtained PVA-methyl polymethacrylate block copolymer in aqueousdispersion with partial precipitates.

A clear film was obtained from this aqueous dispersion by casting.

EXAMPLE 50

Polymerization was performed in the same way as in Example 41, exceptthat No. 9 polymer (PVA having a thiol group at one end of the molecule)was used. Conversion to polymer was 100.5%. There was obtained a 14.7%(solid concentration) aqueous solution of PVA-polyacrylic acid blockcopolymer.

A clear film was obtained from this aqueous solution by casting.

What is claimed is:
 1. A block copolymer in which the components thereofare linked together by divalent sulfur moieties obtained by subjectingthe monomers capable of radical polymerization to radical polymerizationin the presence of a polymer having a thiol group at the end.
 2. A blockcopolymer as set forth in claim 1, which is obtained by performing theradical polymerization under acidic conditions.
 3. The block copolymerhaving a vinyl ester polymer as one component as set forth in claim 1,which is obtained by polymerizing a vinyl monomer composed mainly of avinyl ester in the presence of a polymer having a thiol group at theend.
 4. The block copolymer having a vinyl alcohol polymer as onecomponent as set forth in claim 1, which is obtained by polymerizing avinyl monomer composed mainly of vinyl ester in the presence of apolymer having a thiol group at the end, and then saponifying theresulting polymer.
 5. A block copolymer as set forth in claim 1, whereinthe polymer having a thiol group at the end is polyoxyethylene having aterminal thiol group.
 6. A block copolymer having a vinyl polymer as onecomponent as set forth in claim 1, wherein the polymer having a thiolgroup at the end is a vinyl polymer having a thiol group at one end,said vinyl polymer being represented by the formula P.SH, wherein Pcontains at least one of the constituent units A and B represented bythe following formulas, and having a degree of polymerization lower than3500; ##STR2## wherein R¹ is hydrogen or a C₁₋₆ hydrocarbon group, andR² is hydrogen or a C₁₋₂₀ hydrocarbon group.
 7. A block copolymer havinga vinyl polymer as one component as set forth in claim 1, wherein thepolymer having a thiol group at the end is a vinyl ester polymer havinga thiol group at one end, said vinyl ester polymer being represented bythe formula P.SH, wherein P contains the constituent units A and Brepresented by the following formulas, the content of B being not lessthan 50 mol % and not greater than 100 mol %, and having a degree ofpolymerization lower than 3500; ##STR3## wherein R¹ is hydrogen or aC₁₋₆ hydrocarbon group, and R² is hydrogen or a C₁₋₂₀ hydrocarbon group.8. A block copolymer having a vinyl polymer as one component as setforth in claim 1, wherein the polymer having a thiol group at the end isa vinyl alcohol polymer having a thiol group at one end, said vinylalcohol polymer being represented by the formula P.SH, wherein Pcontains the constituent units A and B represented by the followingformulas, the content of A being not greater than 100 mol % and not lessthan 50 mol %, and having a degree of polymerization lower than 3500;##STR4## wherein R¹ is hydrogen or a C₁₋₆ hydrocarbon group, and R² ishydrogen or a C₁₋₂₀ hydrocarbon group.
 9. A block copolymer having avinyl alcohol polymer as one component as set forth in claim 8, which isobtained by performing radical polymerization in the presence of water.10. A block copolymer as set forth in claim 1, wherein said monomercapable of radical polymerization is at least one monomer selected fromthe group consisting of vinyl esters, acrylic acid, methacrylic acid,acrylic esters, methacrylic esters, acrylamide, methacrylamide,acrylonitrile, methacrylonitrile, vinyl halides, aromatic vinylcompounds and α-olefins.
 11. A block copolymer having a vinyl alcoholpolymer as one component as set forth in claim 6, wherein the monomercapable of radical polymerization is at least one monomer selected fromthe group consisting of acrylic acid, methacrylic acid, acrylic esters,methacrylic esters, acrylamide, methacrylamide, acrylonitrile,methacrylonitrile, vinyl halides, aromatic vinyl compounds, monoolefinsand diolefins.